The invention relates to the standardization of clocks as is required for example in modern communications systems.
In the field of communications, particularly where net-working systems are used, there is frequently a need to acquire and then maintain over long periods of time an accurate time-signal reference to operate electronics communications equipment.
Modern atomic frequency standards are readily available having drift rates of 1 in 10.sup.12 which can maintain an accurate reference over a long period of time without the need for frequent checking or correcting. The difficulty experienced in standardizing distant clocks is to obtain a sufficiently accurate synchronizing time-signal. Generally, current net-working systems require an accuracy of only better than 1 second as they run at relatively low band rates and/or they have been specifically designed to operate from such timing sources. However recent developments in HF/VHF communication systems indicate that timing accuracies within the range of 100 microseconds to 10 milliseconds will be necessary.
The requirement for a time signal source can be met at present using a reasonably stable clock and a standard time signal transmission such as MSF, WWV or GBR etc. The high speed time codes used by these services are transmitted in short bursts one per minute on the minute. The transmitted sequence contains a simple framing preamble followed by a 100 band data code giving GMT and the date. Although in principle a simple serial to parallel conversion is all that is required to decode the information contained in these transmitted codes in practice sophisticated error checking would be essential to discriminate against unwanted signals and to avoid erroneous decoding.
Errors in time code acquisition can occur because of poor propagation or high interference levels which frequently pervade the HF radio spectrum. The accuracy with which the time code signal can be received will depend on the signal to noise ratio of the transmitted time signal, the band width of the receiver and the coding format.